Methods of producing accurate bore surfaces

ABSTRACT

A method of producing a hard, thermal- and wear-resistant bore surface in the inner periphery of a cylinder made of metals, for example, aluminum or its alloy by means of a spray coated layer applied to an accurately finished surface of a hollow cylindrical support member. The spray coated layer applied to the hollow cylindrical support member is used as a hollow mold core. This hollow mold core is incorporated into a casting mold so as to form mold cavity into which is poured molten aluminum or its alloy. The hollow cylindrical support member is removed from the casting to expose the inner surface of the spray coated layer. The exposed inner surface of the spray coated layer is transplanted from the hollow cylindrical support member to the casting and defines an accurate bore surface.

United States Patent [191 Imagawa et al.

[54] METHODS OF PRODUCING ACCURATE BORE SURFACES [76] Inventors:Tameichl Imagawa, No.

85 64-Choma Sakuragi-Cho, Ohmiya', Kaname Kitsuda, No. 28-47Matsukaze-Cho, l-liratsuka, both of Japan [22] Filed: Oct. 18, 1972 [21]Appl. No.: 298,775

[52] US. Cl. 164/95 [51] Int. Cl B22d 19/08 [58] Field of Search 164/94,95, 75

[56] References Cited 7 UNITED, STATES PATENTS 3 40l.736 9/1968 Imagawa164/95 1450.189 6/1969 MacDonald l64/95 Primary Examiner-Robert D.Baldwin Mar. 119, 1974 [57] ABSTRACT A method of producing a hard,thermaland wearresistant bore surface in the inner periphery of acylinder made of metals, for example, aluminum or its alloy by means ofa spray coated layer applied to an accurately finished surface of ahollow cylindrical support member. The spray coated layer applied to thehollow cylindrical support member is used as a hollow mold core. Thishollow mold core is incorporated into a casting mold so as to form moldcavity into which is poured molten aluminum or its alloy. The hollowcylindrical support member is removed from the casting v to expose theinner surface of the spray coated layer.

The exposed inner surface of the spray coated layer is transplanted fromthe hollow cylindrical support member to the casting and defines anaccurate bore surface.

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miminm 1 9 m4 3791556 sum 3 BF 3 F/GL6 METHODS OF PRODUCING ACCURATEBORE SURFACES This invention relates to methods of producing accuratebore surfaces, and more particularly to a method of producing a hard,thermaland wearresistant bore surface in the inner periphery of acylinder made of metals, for example, aluminum or its alloys by means ofa spray coated layer applied to a metal mold core and transplanted tothe metal cylinder.

The method of this kind has been well known as a transplant coatingmethod.

In the conventional method of this kind, use has been made of a solidmetal bar generally made of steel and applied with a spray coated layerso as to construct a metal mold core. The use of such solid metal barshaped metal mold core has a number of disadvantages. In the firstplace, the metal mold core is solid and massive and hence poor inthermal conductivity so that the spray coated layer applied thereon, islocally overheated during spraying thereof and hence there is a risk ofthe spray coated layer being cracked. Secondly, after spraying, thetemperature of the spray coated layer becomes higher than that of themetal mold core, and as a result, during cooling, the spray coated layercould not freely shrink thereby remaining a tensile stress in theperipheral direction thereof. Third, after molten aluminum or alloythereof has been poured in and around the spray coated layer, the spraycoated layer sandwiched between the metal mold core and the casting issubjected to a compressive stress in a direction of the thicknessthereof, and as a result, the spray coated layer is so strongly adheredto the outer surface of the metal mold core that it is extremelydifficult to remove the metal mold core from the spray coated layer.Finally, after the removal of the metal mold core, the spray coatedlayer is released from the tensile stress acting on the pheripheraldirection thereof and freely shrinks to form a gap between the outerperiphery of the spray coated layer and the casting and the gap thusformed having a poor thermal conductivity causes various kind oftroubles to occur which are detrimental to produce the accurate boresurface in the inner periphery of the casting.

An object of the invention is to obviate the above mentioneddisadvantages and to provide an improved transplanting method ofproducing an accurate bore surface with the aid of a hollow cylindricalsupport member applied with a spray coated layer as a metal mold core,by which the hollow cylindrical support member can easily be removedfrom the mold core without deteriorating the spray coated layer.

Another object of the invention is to provide a method of producing suchan accurate cylinder bore surface wherein the bore surface produced isextremely smooth and is of such accuracy that, at most, only a simplehoning operation is required for finishing.

A further object of the invention is to provide a method of producing acast cylinder sleeve for an internal combustion engine which requires noadditional machining beyond a simple honing operation.

Still another object of the invention is to provide a method of applyinga spray coated layer to a hollow cylindrical support member and thentransplanting this spray coated layer onto an article produced in themould by a casting technique.

In the specification, the term accurate bore shall be understood todesignate a bore such, for example, as bearing bores of bimetallicbearing bushes, cylinder bores of internal combustion engines, etc.,which can be formed be removing a hollow cylindrical support member froma spray coated layer permanently mechanically locked to a casting to ahigh degree of accuracy that, at most, only a simple honing operation isrequired for finishing.

According to the invention there is provided a method of producing anaccurate bore surface comprising the steps of applying a spray coatedlayer to the accurately finished outer surface ofa hollow cylindricalsupport member to construct a hollow mold core, said hollow cylindricalsupport member being made of a material having thermal expansioncoefficient and thermal conductivity which are higher than those of saidspray coated layer and having an outer diameter D slightly smaller thanthe inner diameter of said accurate bore surface-and having suchthickness t that a ratio D/: is equal to 8 to 30, inserting said hollowmold core into a casting mold to form an annular cavity between saidhollow mold core and casting mold, pouring molten aluminum or its alloyinto said annular cavity, solidifying said molten aluminum or its alloyto interlock it with the outer surface of said spray coated layer, andremoving said hollow cylindrical support only from said spray coatedlayer interlocked with said aluminum or its alloy, whereby said boresurface is defined by the inner surface of said spray coated layertransplanted from said accurately finished surface of hollow cylindricalsupport member to said metal casting.

In one embodiment of the invention, it is preferable to use as saidhollow cylindrical support member an extruded aluminum pipe.

A preferred embodiment of the invention is illustrated in the followingdrawings, in which:

FIG. 1 is a sectional view of a hollow cylindrical support member whichcan be used to produce an accurate bore surface by the method of theinvention;

FIG. 2 is a sectional view of a hollow mold core constructed by applyinga spray coated layer to the hollow cylindrical support member shown inFIG. 1 with the aid of a metalizing gun;

FIG. 3 is an enlarged fragmentary cross sectional view of the hollowmold core shown in FIG. 2;

FIG. 4 is a sectional view of a casting mold with the hollow mold coreshown in FIG. 2 incorporated therein;

FIG. 5 is an enlarged fragmentary cross sectional view of a castingobtained by the casting mold shown in FIG. 4;

FIG. 6 is a sectional view of a die and punch for removing the hollowcylindrical support member from the hollow mold core; and

FIG. 7 is a cross sectional view of a cast sleeve obtained after theremoval of the hollow cylindrical support member and having an accuratebore surface.

Referring to FIG. 1, reference numeral 1 designates a hollow cylindricalsupport member made of an extruded aluminum pipe and having an outerdiameter D of 38.14 mm and a thickness t of 2.5 mm on the one hand and3.5 mm on the other hand. Thus, a ratio D/t is 15.3 and 10.9,respectively.

In and around the outer periphery of these two kinds of hollowcylindrical support members 1 are formed spray coated layers 2 whosethickness are 0.45 mm and 0.75 mm, respectively. The spray coated layer2 is made of an iron alloy, for example, mild steel and applied in andaround the hollow cylindrical support member 1 to form a hollow moldcore 4 with the aid of a metalizing gun 3 (FIG. 2), the hollowcylindrical support member 1 being rotated about its longitudinal axisduring spraying of the spray coated layer 2.

The spray coated layer 2 thus obtained is porous and is provided at itsexterior with an irregularly pitted and undercut surface and at itsinterior facing the hollow cylindrical support member 1 with a smoothsurface as shown in FIG. 3. During cooling the hollow cylindricalsupport member 1 reduces its diameter and is separated from the spraycoated layer to form a gap on the order of 0.05 mm between the hollowcylindrical support member 1 and the spray coated layer 2 at the coldstate.

Then, the hollow mold core 4 shown in FIG. 2 is incorporated into asplit mold 5 to form a mold cavity between the spray coated layer 2 andthe split mold 5 as shown in FIG. 4. Subsequently, molten aluminum orits alloy is poured through a gate 6 under a pressure of 200 Kg/cm intothe mold cavity to completely fill it with the molten metal 7 and toenter into all of the pits and undercuts on the exposed surface of thespray coated layer 2.

As the molten metal 7 solidifies, it shrinks in and around the spraycoated layer 2 and is interlocked mechanically therewith forming a bondof tremendous strength. Within a few seconds after completion of pouringof the aluminum or its alloy, the two mold halves 5 are separated, andthe casting 7 and the hollow mold core 4 are removed as a unit from thesplit mold 5.

8 designates a mandrel closely fitted in the center cavity of the hollowcylindrical support member 1.

In FIG. 5 is shown an enlarged cross section of the casting 7interlocked with the spray coated layer 2 applied to the hollowcylindrical support member 1. The casting 7 shrinks in a radialdirection to enter into all of the pits and undercuts on the exposedsurface of the spray coated layer 2. Thus, the spray coated layer 2 issubjected to a strong contact pressure for compressing it in a radialdirection, and as a result, the bimetallic casting 7 is subjected to aprestress.

The gap between the spray coated layer 2 and the hollow cylindricalsupport member 1 becomes far smaller than that prior to the castingoperation, but this gap does not prevent removal of the hollowcylindrical support member 1 from the hollow mold core 4.

Eventually, the spray coated layer 2 may locally be adhered to thehollow cylindrical support member 1 whose hardness is smaller than thatof the spray coated layer 2. In such a case, it is preferable to use adie 10 (FIG. 6) which is fitted around the casting 7, for example, ametal sleeve so as to abut its end against a flange 7' of the metalsleeve 7 and then use a punch 11 which serves to apply thrust to thehollow cylindrical support member 1, thereby effecting the removalthereof in an efficient manner.

In order to remove the hollow cylindrical support member 1 from thecasting 7 in a more easy manner, it is preferable to apply, prior to thespraying, onto the outer surface of the hollow cylindrical supportmember 1 a mold releasing agent, for example, a thin separating layerconsisting of a dry lubricant such as synthetic resin, graphite ormolybdenum disulfide, etc.

In FIG. 7 is shown the bimetallic cast sleeve 7 after the hollowcylindrical support member 1 has been removed therefrom, the boresurface is defined by the inner surface of the spray coated layer 2transplanted from the hollow cylindrical support member 1 to the castsleeve 7 and has an inner diameter which is substantially equal to orslightly larger than the outer diameter of the hollow cylindricalsupport member 1 removed. The bore surface of the casting 7 is as smoothas the accurately finished exterior surface of the hollow cylindricalsupport member 1 over which it is formed.

As seen from the above description made with reference to the preferredembodiment shown in FIGS. 1 to 7, in accordance with the invention inthe first place the hollow cylindrical support member 1 is subjected toan accurate machining operation so as to make its outer diameter Dslightly smaller than the inner diameter of the accurate bore to beproduced and make its thickness t comparatively thin such that a ratiobetween the outer diameter D and thickness t D/t is on the order of 8 to30. Moreover, it is required that the hollow cylindrical support member1 is made of a material, for example, aluminum having thermal expansioncoefficient and thermal conductivity which are higher than those of thespray coated layer. In most instances, use may be made of an aluminumpipe extruded from a conventional extruder as the hollow cylindricalsupport member 1. The spray coated layer 2 may preferably be made ofiron alloys, for example, mild steel and carbon steel or copper alloys,etc.

The thermal expansion coefficient of extruded aluminum (28 aluminumalloy) is 23.5 l0' C at a temperature of 20 to C and the thermalconductivity thereof is 0.53 CGS, while the thermal expansioncoefficient at a temperature of 20 to 100 C and thermal conductivity ofthe spray coated layer 2 made of carbon steel are about 1 l.7 lO C andon the order of 0.12 CGS, respectively.

The extruded aluminum pipe with or without subjected to the conventionalanodic oxydation process may be used as the hollow cylindrical supportmember 1.

In case of using the hollow cylindrical support member 1, its outerperiphery may be cleaned, and then the cleaned surface may preferably beformed with a separating layer by spraying a mold releasing agent andsubsequently a material suitable for the spray coated layer, forexample, iron alloy or copper alloy is sprayed thereon.

In case of spraying, the distance between the metalizing gun 3 and thehollow cylindrical support member 1 may preferably be on the order of200 mm to 250 mm. The averate temperature of the spray coated layer 2becomes 100 to C in case of using the hollow cylindrical support member1 made of iron alloy or copper alloy. The temperature of the hollowcylindrical support member 1 having a high thermal conductivity becomesuniformly raised during spraying and all of the region thereof shows notemperature gradient.

After the spraying has been completed, the spray coated layer 2 ispermitted to cool while still in place on the hollow cylindrical supportmember l. The spray coated layer 2 could not follow the contraction ofthe hollow cylindrical support member 1 whose thermal expansioncoefficient is larger than that of the spray coated layer 2 and freelycontracts on the basis of its own thermal expansion coefficient. As aresult, there remains no peripheral tensile stress in the spray coatedlayer 2.

In experimental tests, an extruded aluminum pipe 1 having an outerdiameter of 38.14 mm and thickness of 2.5 mm was spray coated with amild steel layer 2 whose thickness t was 0.5 mm and then the assemblywas cooled to room temperature. These experimental tests have yieldedthe result that the inner diameter of the spray coated layer 2 becomes38.19 mm thereby forming a gap of about 0.05 mm betweenthe extrudedaluminum pipe 1 and the spray coated layer 2, and that there remained noperipheral tensile stress in the spray coated layer 2.

If the hollow mold core 4 as constructed by applying the spray coatedlayer 2 to the outer periphery of the hollow cylindrical support member1 is subjected to a pressure as in the case of a die casting method, themandrel 8 is closely fitted in the hollow mold core as shown in FIG. 4.

In case of applying the conventional casting technique to the hollowmold core 4, the hollow mold core 4 is incorporated into a casting moldand molten aluminum or alloy thereof is poured in the mold cavity. Asthe molten metal solidifies it shrinks in and around the spray coatedlayer 2 and is interlocked mechanically therewith forming a bond oftremendous strength.

In this case, the hollow mold core 4 and more particularly the spraycoated layer 2 applied to the outer periphery of the hollow cylindricalsupport member 1 is heated to substantially the temperature of themolten metal, and as the molten metal solidifies, the spray coated layer2 sandwiched between the hollow cylindrical support member 1 and thecasting 7 whose thermal expansion coefficient is substantially equal tothat of the hollow cylindrical support member 1 acts against theshrinking force radially inwards acting on the casting 7. Thus, thespray coated layer 2 is compressed in the direction of the thicknessthereof and the contact pressure between the spray coated layer 2 andthe metal casting 7 is increased, while the compressive stress in theperipheral direction of the hollow cylindrical support member 1 isremained and permitting free shrinking of the hollow cylindrical supportmember l.

The compressive stress acting in the direction of the thickness of thespray coated layer 2 is remarkably increased, particularly in case ofpressure casting, under the reaction acting on the outer surface of thehollow cylindrical support member 1 supported by the mandrel 8 closelyfitted therein, and as a result, the spray coated layer 2 is compressedto reduce the gap between the compressed fine grains thereof to producea stronger bond between these fine grains thereby significantly improvesthe mechanical property of the spray coated layer 2.

The above mentioned compressive force acting upon the spray coated layer2 is also produced in case of applying no pressure casting technique dueto the reaction caused by the shrinkage of the cast metal 7 and actingupon the surface of the hollow cylindrical support 1 thereby producing astronger bond between fine grains of the spray coated layer 2.

If the contact pressure between the spray coated layer l and the castmetal 7 is increased, the interlock bond between the cast metal 7 andthe fine pits and undercuts on the exposed surface of the spray coatedmolten metal, that is, a prestress is maintained in the spray coatedlayer 2 even under thermal load subjected to castings produced by themethod according to the invention.

As stated hereinbefore, the hollow cylindrical support member ll freelyshrinks as the molten metal 7 solidifies to form a small gap between thehollow cylindrical support member 1 and the spray coated layer 2.

The amount of this gap is far smaller than that of the mold cavityformed between the casting mold andthe hollow mold core 4 incorporatedtherein, but is sufficiently large to permit the hollow cylindricalsupport member 1 only to be removed from the metal casting 7 interlockedwith the spray coated layer 2.

In case of applying the pressure casting technique, the mandrel 8closely fitted in the hollow cylindrical support member 1 limits more orless the free contraction of the hollow cylindrical support member 11.But, the mandrel 8 at its cold state can be removed from the hollowcylindrical support member 1 which can then be removed from the metalcasting 7 interlocked with the spray coated layer 2.

The presence of the gap between the hollow cylindrical support member Iand the spray coated layer 2 makes it possible to remove the hollowcylindrical support member I out of the metal casting 7 and spray coatedlayer 2, even though the compressive force acting upon the outerperiphery of the hollow cylindrical support member 1 is large, with theaid of a punch 11 (FIG. 6) or any other tools. In this case, the spraycoated layer 2 is strongly interlocked with the metal casting 7, so thatthere is no risk of the spray coated layer 2 being damaged.

It is preferable to remove the hollow cylindrical support member 1 withthe aid of the punch Ill or any other pushing tool as mentioned above.But, if necessary, the hollow cylindrical support member I may be cooledat its inner periphery so as to reduce its diameter and hence can beremoved from the spray coated layer 2 applied thereon. Alternatively,the hollow cylindrical support member 1 may be removed by cutting orbroaching operation.

The bore surface of the spray coated layer 2 thus obtained is of asurface transplanted from the accurately finished surface of the hollowcylindrical support member 1. Thus, the outer periphery of the hollowcylindrical support member 1 merely accurately machined makes itpossible to provide an accurate bore with such a high degree of accuracythat only a simple honing operation is required after the castingoperation.

An extruded aluminum pipe has an accurate diameter and smooth surface sothat it may be used as the hollow cylindrical support member 1 withoutrequiring any special working to be subjected thereto.

As described above, in accordance with the invention provision must bemade of a hollow cylindrical support 1 having an outer diameter Dslightly smaller than the inner diameter of an accurate cylinder bore tobe produced and having such a thickness t that a ratio D]! is 8 to 30.

if use is made of a thick hollow cylindrical support member ll whoseratio D/t is smaller than 8, its thermal capacity becomes excessivelylarge. As a result, molten metal particles sprayed thereon is intenselycoated at the surface thereof and hence it becomes difficult to obtain astrong sprayed coated layer 2. Moreover, it is difficult to cool thethick hollow cylindrical support member 1 at a rate earlier than that ofcooling the spray coated layer 2 so that the thick hollow cylindricalsupport member 1 is prevented from freely shrinking, thereby renderingit difficult to attain the object of the invention.

If use is made of a thin hollow cylindrical support member 1 whose ratioD/t is larger than 30, there is a risk of the thin hollow cylindricalsupport member 1 being locally heated by gas flame or gas jet producedupon spraying the molten metal and hence being subjected to a strain.Thus, it becomes difficult to obtain a spray coated layer 2 whosedimensions are of high accuracy. Moreover, such thin hollow cylindricalsupport member 1 is liable to be crushed when it is subjected to theexterior force produced in case of pouring molten metal onto the spraycoated layer 2, and as a result, there is a risk of the castingoperation being failed.

As stated hereinbefore, the method according to the invention makes itpossible to define a bore surface by the inner surface of a spray coatedlayer applied to an accurately finished surface of a hollow cylindricalsupport member and transplanted from it to a casting applied to it andhence to produce an accurate bore surface having such accuracy that, atmost, troublesome machining operations are not required for finishing.

The example is given for the purpose of illustration only and is not tobe considered as limiting the scope of the following claims.

What is claimed is:

l. A method of producing an accurate bore surface comprising the stepsof applying a spray coated layer to the accurately finished outersurface of a hollow cylindrical support member to construct a hollowmold core, said hollow cylindrical support member being made of amaterial having thermal expansion coefficicut and thermal conductivitywhich are higher than those of said spray coated layer and having anouter diameter D slightly smaller than the inner diameter of saidaccurate bore surface and having such thickness t that a ratio D/t isequal to 8 to 30, inserting said hollow core mold into a casting mold toform an annular cavity between said hollow mold core and casting mold,pouring molten aluminum or its alloy into said annular cav-

1. A method of producing an accurate bore surface comprising the stepsof applying a spray coated layer to the accurately finished outersurface of a hollow cylindrical support member to construct a hollowmold core, said hollow cylindrical support member being made of amaterial having thermal expansion coefficient and thermal conductivitywhich are higher than those of said spray coated layer and having anouter diameter D slightly smaller than the inner diameter of saidaccurate bore surfAce and having such thickness t that a ratio D/t isequal to 8 to 30, inserting said hollow core mold into a casting mold toform an annular cavity between said hollow mold core and casting mold,pouring molten aluminum or its alloy into said annular cavity,solidifying said molten aluminum or its alloy to interlock it with theouter surface of said spray coated layer, and removing said hollowcylindrical support only from said spray coated layer interlocked withsaid aluminum or its alloy, whereby said bore surface is defined by theinner surface of said spray coated layer transplanted from saidaccurately finished surface of hollow cylindrical support member to saidmetal casting.
 2. A method as claimed in claim 1 wherein said hollowcylindrical support member is an extruded aluminum pipe.